Historically, halide-containing raw materials, such as, SiCl.sub.4 or mixtures of SiCl.sub.4 with various dopants, have been used in the manufacture of preforms by vapor phase deposition techniques, such as, the MCVD (modified chemical vapor deposition), VAD (vapor axial deposition), and OVD (outside vapor deposition) techniques.
In the MCVD technique, the halide-containing raw materials are vaporized and reacted with oxygen to form oxide particles which are deposited on the inside of a fused-silica tube. In the VAD and OVD procedures, vaporized, halide-containing raw materials are hydrolyzed in a burner to produce soot particles which are collected on a rotating starting rod (bait tube) in the case of VAD or a rotating mandrel in the case of OVD. In some OVD systems, the cladding portion of the preform is deposited on a previously-formed core preform, rather than on a mandrel.
Various vaporizers have been developed which can be used in such processes, examples of which can be found in Blankenship, U.S. Pat. No. 4,314,837, French, U.S. Pat. No. 4,529,427 (flash vaporizer), Tsuchiya et al., U.S. Pat. No. 4,938,789, Antos et al., U.S. Pat. No. 5,078,092 (halide-free reactants), and Japanese Patent Publication No. 58-125633 (flash vaporizer). Soubeyrand et al., U.S. Pat. No. 5,090,985, discloses the use of a horizontal thin film evaporator for vaporizing various raw materials employed in the preparation of coated glass articles.
Aslami, U.S. Pat. No. 4,212,663, discloses a vaporization system for a liquid reactant, e.g., SiCl.sub.4, in which a heated reactant is deposited on the top of a column containing a porous packing material and carrier gas, e.g., oxygen, is bubbled through a reservoir of the liquid reactant and then up through the column. The Aslami system can include a second, heated column located above the first column which serves to: (1) ensure that the carrier gas is fully saturated, and (2) prevent droplets of the liquid reactant from being entrained in the flowing gas. Significantly, with regard to the present invention, Aslami's reactant and carrier gas do not flow downward together to achieve vaporization and then change direction together to separate out undesirable higher molecular weight species.
U.K. Patent Publication No. 1,559,978 shows another system employing a porous material in the vaporization of liquid reactants, including SiCl.sub.4. In this case, the porous material is placed in a vessel containing the liquid reactant, hot silicone oil is flowed over the outside surface of the vessel to heat the reactant, and carrier gas is bubbled through the reactant or introduced into a portion of the porous material located above the upper surface of the reactant. The porous material is said to promote differential evaporation between the liquid reactant and impurities contained therein, whereby a purer gas stream is said to be achieved by using either the early part of the gas stream in the case of an impurity having a vapor pressure lower than that of the reactant or a later part of the stream in the case of an impurity having a vapor pressure higher than that of the reactant.
As with the Aslami patent, this U.K. patent publication does not disclose or suggest the vaporization system of the present invention in which specific flow patterns are used to vaporize a polymerizable material and to separate higher molecular weight species from the vapor stream.
The use of halide-containing raw materials generates substantial quantities of halide-containing by-products, e.g., hydrochloric acid. To avoid environmental pollution, these by-products must be collected, which increases the overall cost of the preform production process. Accordingly, halide-free materials and, in particular, halide-free, silicon-containing materials are desirable starting materials for the production of preforms. See Dobbins et al., U.S. Pat. No. 5,043,002.
As explained in the Dobbins et al. patent, the relevant portions of which are incorporated herein by reference, particularly preferred halide-free, silicon-containing materials for use in producing preforms are polymethylsiloxanes, with the polymethylcyclosiloxanes being particularly preferred, and with octamethylcyclotetrasiloxane (OMCTS) being especially preferred. These same halide-free, silicon-containing raw materials are preferred for use with the present invention.
Cain et al., U.S. Pat. No. 5,356,451, discloses a vaporizer specifically developed for use with halide-free, silicon-containing raw materials, such as OMCTS. In accordance with this technology, a preheater in series with a flat plate vaporizer is used to change the state of the raw material from a liquid to a vapor. Both the preheater and the flat plate vaporizer use electrical resistance wires as their source of heat. In the preheater, the liquid is heated to below its boiling point. In the flat plate vaporizer, additional heat is added to (1) bring the fluid to its boiling temperature and (2) provide the necessary energy to overcome the material's latent heat of vaporization, whereupon the fluid becomes a vapor.
Halide-free, silicon-containing materials are difficult to vaporize due to their sensitivity to cracking and polymerization when subjected to excessive temperatures. Polymerization results in the production of higher molecular weight species which form gums and gels. Additionally, higher molecular weight species may exist in the raw materials as originally manufactured. Such species do not easily vaporize and over time the polymerized materials tend to foul heat transfer surfaces and plug piping systems. Current data suggest that it may not be possible to produce vapor from materials such as OMCTS without incurring some polymerization.